From what im gathering on this it
could be the Delco GM 2700 ETR Radio. I assume it doesn't have built in CB?
Does it look like this:
This page gives a wiring idea:
http://www.transamradio.com/ta_radio_004.htm
According to that the Orange wire supplies power to the clock and dimmer. (Which normally orange does go to your dimmer)
Lastly if that is the radio, from what I'm seeing it looks like its one of the tougher Delco's to find.